A computational and experimental study of turbulent partially premixed jet flames

Turbulent partially premixed flames in a coflow of air are studied for possible application to low NO_xcombustors. Lean Direct Injection (LDI) involves direct injection of fuel in the primary air flow upstream of the flame zone possibly leading to partially premixed flames. The structure of flames that leads to low NO_x emissions in LDI systems has been debated in the literature. Experimental studies of laminar methane and propane fueled partially premixed flames have shown that an optimum burner tube equivalence ratio exists for low NO_x emissions.¹ Measurements of gas species concentrations showed a rich premixed inner flame and an outer diffusion flame for equivalence ratios close to the optimum. This paper presents results from a computational and experimental study to examine if turbulent partially premixed flames also involve a double-flame structure. The existence of an optimum fuel tube equivalence ratio for minimization of NO_x emissions was verified experimentally for turbulent flames with a fixed fuel flow rate. Computational predictions of partially premixed flames also reveal a doubie-flame structure in qualitative agreement with the experimental results.